Cover glass for display device, and manufacturing method for same

ABSTRACT

The present invention aims to provide a cover glass for display devices, made of soda-lime glass, excellent in cutting easiness and reliability of surface strength. The cover glass for display devices of the present invention includes a chemically strengthened glass, and has a compressive stress layer having a depth of 6 to 15 μm. 
     In the cover glass, a shape parameter determined in accordance with JIS R 1625 (1996) based on analysis of a facture stress of the cover glass measured by a coaxial double ring test is not less than 7, and strength of the cover glass when a cumulative fracture probability is 1% is not less than 450 MPa. 
     The glass plate before the ion exchange is made of soda-lime glass.

TECHNICAL FIELD

The present invention relates to a cover glass for display devices,specifically a chemically strengthened glass suitable for cover glassesor integrated cover glasses having functions of both a substrate and acover glass for display devices (including display devices havingfunctions of an input arrangement) of electric devices (e.g. mobilephones, smartphones, tablet computers).

BACKGROUND ART

Resin covers are widely used as display protectors for mobile electronicdevices such as mobile phones and smartphones. Such resin covers,however, are exceeded by those made of glass in terms of excellence intransmittance, weather resistance, and damage resistance, andadditionally, glass improves the aesthetics of displays. Accordingly,there has been an increasing demand for display protectors made of glassin recent years. A cover glass is a component that has an exposedsurface, and therefore is susceptible to cracking when exposed to animpact (e.g. contact with a hard object, dropping impact). For thisreason, a demand for glasses with high mechanical strength has beenraised.

Two methods of strengthening glass plates are known. One is thermalstrengthening (physical strengthening) which involves heating a surfaceof a glass plate nearly to the softening point of the glass plate andrapidly cooling the surface with a cool blast or the like.Unfortunately, thermal strengthening, when performed on a thin glassplate, is less likely to establish a large temperature differentialbetween the surface and the inside of the glass plate, and thereforeless likely to provide a compressive stress layer at the surface portionof the glass plate. Thus, this method fails to provide desired highstrength. Another fatal problem is that processing (e.g. cutting) of athermally strengthened glass plate is difficult because the glass platewill shatter when a preliminary crack for cutting is formed on thesurface.

The other method is chemical strengthening. The chemical strengtheningmay be used for strengthening a thin glass plate to be used as a coverglass or a glass with a complicated shape. Chemical strengtheninginvolves ion exchange of a glass to form a compressive stress layer atthe surface layer of the glass. For example, low-temperature typechemical strengthening is widely known in which a glass made ofsoda-lime glass or the like is submerged in a molten salt (e.g.potassium nitrate) at a temperature lower than the annealing point ofthe glass to perform ion exchange of alkali metal ions (e.g. sodiumions) having a smaller ionic radius present in a surface layer of aglass with alkali metal ions (e.g. potassium ions) having a larger ionicradius.

Surface compressive stress and depth of a compressive stress layer canbe used as measures of the strength of chemically strengthened glasses.The term “surface compressive stress” or simply “compressive stress”refers to a compressive stress in the outermost layer of a glass, whichis generated by incorporation of ions having a larger volume into thesurface layer of the glass by ion exchange. A compressive stress cancelstensile stress that is a factor of breaking glasses, and thuscontributes to higher strength of chemically strengthened glasses thanthat of other glasses. Accordingly, the surface compressive stress canbe used as a direct measure for the improvement of the strength ofglasses.

The “depth of a compressive stress layer” or simply “depth of layer”refers to the depth of a region where a compressive stress is present,as measured from the outermost surface of the glass plate as a standard.A deeper compressive stress layer corresponds to higher ability toprevent a large microcrack (crack) on the surface of the glass fromgrowing, in other words, higher ability to maintain the strength againstdamage.

In order to efficiently perform ion exchange of a surface layer of aglass, that is, in order to form a deeper compressive stress layer, theglass needs to be in contact with a molten salt at higher temperaturesor for a longer period of time. However, this also causes an increase ina relaxation rate of a compressive stress derived from the viscosity ofthe glass. For this reason, the compressive stress generated by ionexchange tends to be high when the glass is allowed to be in contactwith a molten salt at lower temperatures or for a shorter period oftime. In general, a surface compressive stress and a depth of acompressive stress layer cannot keep both the sufficient levels, and itis difficult to find production conditions suitable for chemicalstrengthening.

The following chemically strengthened glasses and methods ofmanufacturing the chemical strengthening glasses have been reported sofar. For example, Patent Literature 1 has disclosed a chemicallystrengthened glass improved in glass strength by, as a primarytreatment, contacting main alkali metal ions A, which are the largest inamount among all the alkali metal ion components of the glass, with asalt that contains only the alkali metal ions A to increase the amountof the alkali metal ions A in a surface layer; and, as a secondarytreatment, replacing the alkali metal ions A with alkali metal ions Bhaving a larger ionic radius than the alkali metal ions A. PatentLiterature 2 has disclosed a method of chemical strengthening. Themethod includes, as a primary treatment, contacting a soda-lime-basedglass plate with a salt at a temperature equal to or lower than thestrain point of the glass plate for an appropriate period of time. Here,the salt contains alkali metal ions A and alkali metal ions B having alarger ionic radius than the alkali metal ions A such that theproportion of the alkali metal ions A should meet a proportion P (whichis a proportion of the alkali metal ions A to the total amount of thealkali metal ions A and B). The methods subsequently includes, as asecondary treatment, contacting the glass plate with a salt having aproportion Q, which is smaller than the proportion P, under at least onecondition selected from conditions where a temperature is lower than thetemperature in the primary treatment and a time period is shorter thanthat in the primary treatment. Further, Patent Literature 3 hasdisclosed a method of strengthening aluminosilicate glass which includesprimary and secondary treatments.

Soda-lime glass is widely used as a material of glasses used in achemically strengthened form, such as a cover glass. Soda-lime glass isextremely commonly used for a glass plate. Further, soda-lime glass is alow-cost glass suitable for mass production, and is therefore usedwidely in various applications. An aluminosilicate composition, whichhas a higher ion exchange capacity than soda-lime glass, tends to beused for cover glasses used as a protective member for recent displays.For example, Patent Literatures 4 and 5 have disclosed analuminosilicate composition designed to be chemically strengthened.

As described above, glasses are in high demand in protective members fordisplay devices of mobile electronic devices (e.g. mobile phones,smartphones, tablet computers). Display devices with a touch panelfunction explosively increase their market share. Such display devicesgenerally include a liquid crystal panel as an information displaydevice, a touch panel as an input arrangement, and a cover glass forprotecting these. Such a cover glass is conventionally manufactured bycutting and processing a large glass plate into glasses in a certainshape, and subsequently chemical strengthening the respective glasses.Such a conventional method needs two or more of glass members for atouch panel substrate and a cover glass.

Regarding a recent manufacturing manner of touch panels, an integratedcover glass has been developed which is a single cover glass equippedwith touch sensors thereon having functions of both a cover glass and atouch panel. Manufacturing of such an integrated cover glass includesforming touch sensors on a chemically strengthened large glass plate,and subsequently cutting the glass plate into cover glasses in a certainshape. This manufacturing method of integrated cover glasses greatlydiffer from the conventional manufacturing method in that the integratedcover glasses are prepared by cutting a chemically strengthened glass.In this case, cutting processability of a chemically strengthened glassis required.

Chemically strengthened glasses can be cut, but with great difficulty.Cut difficulty of chemically strengthened glasses is a main reason forreducing the production yield, and also causes breakage or otherproblems of products made from chemically strengthened glasses. In orderto solve the above problems, Patent Literatures 6 and 7 have disclosed asoda-lime-based chemically strengthened glass which is appropriatelycut.

Regarding mechanical strength (e.g. resistance to contact with a hardobject, dropping impact) of glass, glass is a brittle material. Abrittle material breaks due to growth of a microcrack (crack) on thesurface of the material. The density and the size of microcracks arestochastically-distributed on the surface of the material. Therefore,the strength of a brittle material is stochastically-distributed widely.That is, the strength of a glass material is not represented as aproperty inherent in the material, and essentially varies. For thisreason, in order to obtain a cover glass reliable as a product, thecover glass needs to be improved in strength based on the control of thestatistical distribution. A measure of such strength is, for example, astrength value when a cumulative fracture probability is 1%(hereinafter, strength when a cumulative fracture probability is 1%).

In order to improve the mechanical strength with strength reliability,resistance to impacts and damages that cause breakage of glasses wouldgenerally be enhanced by increasing a surface compressive stress and bymaking a deeper compressive stress layer. However, this means that itdoes not allow even a preliminary crack for glass cutting to be formedthereon, which causes difficulty in cutting. In addition, a high surfacecompressive stress and a deeper compressive stress layer generate a highcentral tensile stress in the inside of a glass. Even if a crack forcutting can be formed on the glass, the central tensile stressautomatically propagates the crack made for cutting to cause a danger ofshuttering the glass. Thus, high strength reliability tends to impairthe cutting easiness of a chemically strengthened glass.

As described above, in a chemically strengthened glass, good cuttingeasiness is contrary to good strength reliability. They are thereforedifficult to keep both in a balanced manner. Accordingly, a cover glassmade of a chemically strengthened glass having not only excellentmechanical strength with strength reliability (strength when acumulative fracture probability is 1%), but also excellent cuttingprocessability, is needed.

CITATION LIST Patent Literature

-   Patent Literature 1: JP H8-18850 B-   Patent Literature 2: JP S54-17765 B-   Patent Literature 3: JP 2011-529438 T-   Patent Literature 4: JP 2010-275126 A-   Patent Literature 5: JP 2011-213576 A-   Patent Literature 6: JP 2004-359504 A-   Patent Literature 7: JP 2004-83378 A

SUMMARY OF INVENTION Technical Problem

As described above, it is usual to require higher mechanical strengthagainst impacts (e.g. contact with a hard object, dropping impact)needed for cover glasses as well as cutting easiness of a chemicallystrengthened glass for good productivity of cover glasses. The importantfactor in the mechanical strength is finally the strength of a glasssurface, that is, the surface strength, because a side surface (edge)part of a cover glass attached to a display device of an electronicdevice is usually protected. Here, it is known that the strength of anedge (side surface) depends only on the shape of the edge afterprocessing, and does not contribute to the measure of the strength as aglass material. However, it is difficult to provide a cover glasssuitable for mass production at low costs, having high strength when acumulative fracture probability is 1%, and made of a chemicallystrengthened glass with good cutting processability, by onlyconventional inventions.

Soda-lime glass is not suitable for chemical strengthening that involvesion exchange of a surface layer of a glass although it has been used asa material for windowpanes, glass bins, and the like, and is a low-costglass suitable for mass production. Patent Literatures 4 and 5 havedisclosed glass (aluminosilicate glass) suitable for chemicalstrengthening in view of chemical composition. Aluminosilicate glass isdesigned to have a higher ion exchange capacity than soda-lime glass by,for example, increasing the amount of Al₂O₃ to substantially 10% (mass%) or more, which improves the ion exchange capacity, and adjusting theratio between alkali metal oxide components Na₂O and K₂O or the ratiobetween alkaline-earth metal oxide components MgO and CaO, and thus isoptimized for chemical strengthening. Aluminosilicate glass, which hashigher ion exchange capacity than soda-lime glass as described above, isable to form a deep compressive stress layer having a depth of 20 μm ormore, or a deeper depth of 30 μm or more. A deep compressive stresslayer has high damage resistance, but unfortunately, this means that itdoes not allow even a preliminary crack for glass cutting to be formedthereon. Even if a crack can be formed on the glass, it is impossible tocut the glass along the crack, and if a deeper crack is formed, theglass may shatter. Thus, it is very difficult to cut a chemicallystrengthened aluminosilicate glass.

As described above, a chemically strengthened aluminosilicate glass hastoo high a surface compressive stress and too deep a compressive stresslayer. This means that the aluminosilicate glass has good reliabilityfor required strength, but has high difficulty in cutting. Therefore, itis hard to say that such an aluminosilicate glass is suitable for anintegrated cover glass. Even if the problem of cutting were overcome,aluminosilicate glass requires a higher melting temperature thansoda-lime glass because aluminosilicate glass contains larger amounts ofAl₂O₃ and MgO, which elevate the melting temperature, than thosecontained in soda-lime glass. In a mass production line, it is producedvia a highly viscous molten glass, which leads to poor productionefficiency and high costs.

Accordingly, there is a demand for a technique enabling use of soda-limeglass, which is widely used for glass plates, is more suitable for massproduction than aluminosilicate glass, and therefor is available at lowcost, and is already used in various applications, as a glass material.However, it would be difficult to provide a cover glass having bothsufficient strength and good cutting easiness by treating soda-limeglass in a conventional manner because higher strength contrary enhancescutting difficulty.

The cutting easiness and the strength of a chemically strengthenedsoda-lime glass are described below.

As for cutting easiness, Patent Literatures 6 and 7 have disclosedchemically strengthened glasses suitable for cutting. However, PatentLiterature 6 has focused only on the surface hardness among variousproperties of a chemically strengthened glass, but is silent on asurface compressive stress and a depth of a compressive stress layer,which are important properties of a chemically strengthened glass.Patent Literature 7 has reported a surface compressive stress and adepth of a compressive stress layer, but these are similar levels tothose of general chemically strengthened glass articles. Therefore, thesurface compressive stress of soda-lime glass will not be likely to beimproved based on the teachings in Patent Literature 7. Thus, soda-limeglass has relatively good cutting easiness compared to aluminosilicateglass, but is difficult to increase the strength.

Patent Literatures 1 to 3 have disclosed conventional techniques forimproving strength. Improvement in the strength of a chemicallystrengthened glass in Patent Literature 1 is characterized mainly by theprimary treatment, that is, a step of contacting a glass plate with asalt that contains only Na ions, which are a main ionic component of theglass plate. In this method, since the amount of Na ions, which are tobe exchanged, in a surface layer of a glass is increased through theprimary treatment, a compressive stress generated by exchanging Na ionswith K ions will be increased in the secondary treatment. The presentinventors have studied on improvement of the strength and the cuttingeasiness of a chemically strengthened glass based on the teachings ofPatent Literature 1, and found some points to be solved. Specifically,there is room for improvement in reducing stress relaxation occurred byperforming exchange of Na ions with K ions in the secondary treatment.Further, the statistical distribution of the strength of a chemicallystrengthened glass has not been examined yet, and the strengthreliability of a glass as a product has not been clearly disclosed.Further, cutting easiness of a chemically strengthened glass has notbeen examined yet. In addition, the primary treatment allows to increasethe amount of Na ions in a surface layer of a glass, which are to beexchanged. However, the surface of the glass brought in contact with toomuch Na ions in the primary treatment is likely to become cloudy tocreate a large problem on improving strength reliability.

Patent Literature 2 has disclosed a method of chemical strengtheningwhich allows improvement of glass strength. However, the conditions ofthe chemical strengthening satisfying the requirement in the PatentLiterature 2 include huge combinations of various conditions. Inaddition, the statistical distribution of the strength has not beenexamined yet, and the strength reliability of a glass as a product hasnot been clearly disclosed. The soda-lime-based chemically strengthenedglass prepared according to Example 1 in Patent Literature 2 isdifficult to cut. According to Example 1, it required a lot of time forcarrying out the primary and secondary treatments. Therefore, the methodis not suitable for realistic mass production.

Patent Literature 3 has disclosed a method of chemical strengthening toavoid an influence of ions eluted from the surface of a glass asimpurities in a molten salt bath. The method can solve the problem ofgradual dilution of the salt bath by the eluted ions. Such chemicalstrengthening is a method continuously producing chemically strengthenedglasses having the same level of strength as that of a chemicallystrengthened glass prepared in an uncontaminated salt bath. However, themethod is not a method for producing a glass having more improvedstrength than that of a glass prepared in an uncontaminated salt bath.

It has been understood that a glass plate which can be cut and has arelatively high surface compressive stress of about 600 MPa can be madeof soda-lime glass even by one-step chemical strengthening, which doesnot include the primary and secondary treatments, disclosed in PatentLiteratures 1 to 3, under the conditions where alkali ions (that is, Kions) having a larger ionic radius in a salt bath are ultra-highlypurified and the depth of surface compressive stress layer is set at 10to 13 μm so as to be cut by a scribing treatment. However, it wasrecently found that the strength when a cumulative fracture probabilitywas 1% determined in accordance with JIS R1625 (1996) from analysis ofthe statistical distribution of the surface strength of the glass platewas less likely to increase. That is, it was found that the glass had ahigh surface compressive stress, but had poor strength reliability.

Thus, several problems lain in the conventional manners have beendescribed above. It is hard to say that technical studies have been madeso far on development for a cover glass with sufficient strengthrelating to the strength reliability when a cumulative fractureprobability is 1% and cutting easiness, made of soda-lime glass, whichhas a lower ion exchange capacity than that of aluminosilicate glass andis not particularly suitable for chemical strengthening in view ofcomposition.

In order to solve the above conventional problems, the present inventorshave extensively examined on a chemically strengthened glass havingexcellent cutting processability and a high surface compressive stress.Consequently, the inventors have achieved the invention of a glass platewhich can be cut and has a high surface compressive stress, highstrength when a cumulative fracture probability is 1%, and excellentstrength reliability, produced from a chemically strengthened soda-limeglass.

That is, the present invention provides a cover glass for displaydevices and a method of manufacturing the cover glass. The cover glassis made of soda-lime glass, is suitable for cover glasses for displaydevices, and can be easily cut. Further, the cover glass has highsurface strength when a cumulative fracture probability is 1%, that is,has excellent strength reliability.

Solution to Problem

A cover glass for display devices of the present invention includes

a chemically strengthened glass manufactured by ion exchange of asurface layer of a glass plate to replace Na ions with K ions,

the cover glass having a main surface part in which a compressive stresslayer is formed and a side surface part that has a region where thecompressive stress layer is formed and a region where no compressivestress layer is formed,

the compressive stress layer having a depth of 6 to 15 μm,

wherein a shape parameter determined in accordance with JIS R 1625(1996) based on analysis of a facture stress of the cover glass measuredby a coaxial double ring test is not less than 7, and strength of thecover glass when a cumulative fracture probability is 1% is not lessthan 450 MPa; and

the glass plate before the ion exchange is made of soda-lime glasssubstantially composed of SiO₂: 65 to 75%, Na₂O+K₂O: 5 to 20%, CaO: 2 to15%, MgO: 0 to 10%, and Al₂O₃: 0 to 5% on a mass basis.

The cover glass for display devices of the present invention includes achemically strengthened glass in which Na ions in the surface layer ofthe glass are ion exchanged with K ions present outside the glass. Sincethe K ions are incorporated in the structure of the surface layer, thesurface layer tends to have a volume expansion. Under the temperatureconditions in this method, the glass cannot reduce the volume expansion.Consequently, the expansion substantially remains as a residualcompressive stress in the glass. The cover glass for display devices ofthe present invention is prepared by cutting a glass plate for a coverglass for display devices of the present invention. The cover glass thusobtained has side surface parts (edge parts) that have a region where acompressive stress layer is formed and a region where no compressivestress layer is formed.

In the cover glass for display devices of the present invention, thedepth of the compressive stress layer, with a compressive stressobtained through ion exchange, in the main surface part and the sidesurface part is 6 to 15 μm. A glass having a compressive stress layerwith a depth of less than 6 μm cannot withstand commercial use becausemicrocracks may be formed in use and such microcracks reduce thestrength of the glass. On the other hand, a glass having a compressivestress layer with a depth of more than 15 μm may be difficult to cut byscribing.

The most important feature of the cover glass for display devices of thepresent invention is that it has a compressive stress layer with alimited depth, and has improved reliability of the mechanical strengthas a cover glass. As a measure of the reliability of the mechanicalstrength of the cover glass, a shape parameter obtained in accordancewith JIS R1625 (1996) by statistically processing, the surface strengthmeasured by a coaxial double ring test, and a strength when a cumulativefracture probability is 1% are evaluated. A shape parameter (Weibullmodulus) is a measure of variation of strength distribution of a brittlematerial. The strength needed for the cover glass for display devicesis, for example, set at not less than 400 MPa in a productspecification. Thus, design strength including certain safety isrecognized. In the cover glass for display devices of the presentinvention, “the strength when a cumulative fracture probability is 1% is450 MPa” means that even if an external action is applied to a coverglass so that 450 MPa of an internal force is generated, the fractureprobability is no more than 1%. A cover glass having a shape parameterof less than 7 or strength when a cumulative fracture probability is 1%of less than 450 MPa is poor in strength reliability, and thereforecannot withstand commercial use.

The cover glass for display devices of the present invention includes aglass made of soda-lime glass having a specific composition as a glassbefore ion exchange. Use of soda-lime glass is advantageous compared touse of aluminosilcate glass suitable for chemical strengthening becausean increase in production costs due to material change and reducedproduction efficiency is avoided. For example, to increase the amount ofaluminum oxide in a composition (e.g. the design of the composition ofaluminosilicate glass) is effective for increasing the ion exchangecapacity, but is accompanied by not only increased material costs butalso remarkable elevation of the melting temperature of the glass, whichcontributes to remarkably high production costs of the glass. Anothereffective way to increase the ion exchange capacity is to use MgO as thealkaline-earth component in place of a portion of CaO. This, however,also elevates the melting temperature of the glass, and thereby leads toincreased production costs.

In the cover glass for display devices of the present invention, themain surface part preferably has a surface compressive stress of notless than 450 MPa.

A glass having a surface compressive stress of less than 450 MPa hasreduced average strength, and is susceptible to breaking when exposed toan impact (e.g. contact with a hard object, dropping impact).

In the cover glass for display devices of the present invention, the ionexchange preferably includes a first step of contacting the glass platewith a first salt that includes Na ions and K ions at a proportion P ofthe Na ions as expressed as a molar percentage of the total amount ofthe Na ions and the K ions; and a subsequent second step of contactingthe glass plate with a second salt that includes Na ions and K ions at aproportion Q of the Na ions as expressed as a molar percentage of thetotal amount of the Na ions and the K ions, where the proportion Q issmaller than the proportion P.

A glass plate for a cover glass for display devices of the presentinvention includes

a chemically strengthened glass plate manufactured by ion exchange of asurface layer of a glass plate to replace Na ions with K ions, the glassplate having a compressive stress layer with a depth of is 6 to 15 μm,

wherein a shape parameter determined in accordance with JIS R 1625(1996) based on analysis of a facture stress of the glass plate measuredby a coaxial double ring test is not less than 7, and strength of theglass plate when a cumulative fracture probability is 1% is not lessthan 450 MPa, and

the glass plate before the ion exchange is made of soda-lime glasssubstantially composed of SiO₂: 65 to 75%, Na₂O+K₂O: 5 to 20%, CaO: 2 to15%, MgO: 0 to 10%, and Al₂O₃: 0 to 5% on a mass basis.

A method of manufacturing the cover glass for display devices of thepresent invention includes the steps of:

performing ion exchange of a surface layer of a glass plate to replaceNa ions with K ions; and

cutting the glass plate,

the ion exchange including a first step of contacting the glass platewith a first salt that includes Na ions and K ions at a proportion P ofthe Na ions as expressed as a molar percentage of the total amount ofthe Na ions and the K ions; and

a subsequent second step of contacting the glass plate with a secondsalt that includes Na ions and K ions at a proportion Q of the Na ionsas expressed as a molar percentage of the total amount of the Na ionsand the K ions, where the proportion Q is smaller than the proportion P.

A brittle material such as glass is susceptible to breaking at a locallyweakened portion of the surface when an external force is applied to thesurface. In the present invention, use of the above first salt, that is,use of a salt containing Na ions and K ions in only certain amountsallows to proceed ion exchange to some extent in the first step, wherebythe surface layer of the glass is modified so as to contain both the Naions and the K ions. It is considered that the virtual temperature ofthe glass may decrease to make the structure of the glass tight (dense)in this first step. It is considered that, in the subsequent second stepin which a salt having a proportion Q, which is smaller than theproportion P, is used, ion exchange between the Na ions and the K ionsoccurs in the outermost surface of the glass, and, in addition, the Kions are three-dimensionally redispersed in the surface layer of theglass modified through the first step. In this second step, a highsurface compressive stress may generate, and microcracks on the surfaceof the glass may be prevented from growing.

The following experimental data supports that the tight glass structureis obtained by the decrease of the virtual temperature in the firststep. The average densities of a glass plate not chemicallystrengthened, a glass plate prepared through one-step chemicalstrengthening, and a glass plate prepared through two-step chemicalstrengthening are measured to be 2.489 g/cm³, 2.493 g/cm³, and 2.497g/cm³, respectively. The densities of the glass plates prepared throughchemical strengthening are measured after a strengthened layer isremoved. The results show that the glass plate prepared through two-stepchemical strengthening has a higher density.

In the ion exchange in the chemical strengthening, a surface compressivestress and a depth of a compressive stress layer of a chemicallystrengthened glass are affected by the temperature and the period oftime on the chemical strengthening, and the type of a selected treatmentliquid and the active property of the treatment liquid. Further, asurface compressive stress and a depth of a compressive stress layer ofa chemically strengthened glass may depend on the state of ion exchangein the glass. In particular, in the case of conventional one-stepchemical strengthening, a surface compressive stress and a depth of acompressive stress layer are in a trade-off relationship, and aredifficult to keep both to sufficient levels. On the contrary, two-stepchemical strengthening (ion exchange) and appropriate selection oftemperature, period of time on treatment, and composition of thetreatment liquid make it possible to effectively enhance the effect ofeach step, thereby producing a chemically strengthened glass which canbe cut and has a high surface compressive stress.

According to the ion exchange described above, the composition of thesurface layer of the glass is modified by ion exchange of the Na ionswith the K ions in the first step, while the Na ions, which contributeto generation of a compressive stress, are left in the layer. The amountof the K ions is larger in the surface layer modified in the first stepthan in the layer before the first step. Therefore, the surface layermodified in the first step has a higher strain point. As a result, therelaxation of the stress generated in the second step can be prevented.Thus, a chemically strengthened glass with a high surface compressivestress, made of soda-lime glass, can be produced.

In the method of manufacturing the cover glass for display devices ofthe present invention, the compressive stress layer formed at a surfaceof the glass plate through the first step preferably has a depth of 5 to23 μm.

If the depth of the compressive stress layer formed through the firststep is too small, the composition of the surface layer of the glass isnot sufficiently modified in the primary treatment, whereby the stressrelaxation occurred in the secondary treatment may not be sufficientlyprevented. On the contrary, if the depth of the compressive stress layerformed through the first step is too large, the depth of the compressivestress layer finally formed through the secondary treatment becomeslarge, which adversely affects the cutting easiness of the glass.

As described above, the stress relaxation in the secondary treatment canbe prevented by performing the primary treatment, in the presentinvention. However, glass is inherently impossible to completely preventprogress of stress relaxation. Therefore, a slight stress relaxation mayoccur in the secondary treatment, and thus the depth of the compressivestress layer finally remaining after the secondary treatment may bechanged from the depth of the compressive stress layer formed throughthe primary treatment. On the contrary, it may be assumed that theamount of ions exchanged in the secondary treatment is larger than thatof ions exchanged in the primary treatment, and the depth of thecompressive stress layer formed through the second step is slightlydeeper than the depth of the compressive stress layer obtained throughthe primary treatment. However, the depth of the compressive stresslayer finally formed through the second step is only slightly changedfrom the depth of the compressive stress layer formed through the firststep (the primary treatment). Since the cutting easiness of the obtainedchemically strengthened glass is significantly affected by the depth ofthe compressive stress layer formed through the first step, it isimportant to control the depth of the compressive stress layer formedthrough the first step.

Thus, the glass obtained through the first step preferably has acompressive stress layer with a depth of 5 to 23 μm at the surfacethereof.

In relation to the depth of the compressive stress layer formed throughthe first step, the temperature of the first salt and the period of timefor contact of the glass plate with the first salt are controlleddepending on the proportion P in the first salt.

If the proportion P in the first salt is too high, the surface of theglass plate is likely to become cloudy, and improvement in thereliability of the glass strength is avoided. On the contrary, if theproportion P is too low, the composition of the surface layer of theglass plate tends to be too sufficiently modified, whereby most of theNa ions in the glass are ion exchanged with the K ions in the firststep. For this reason, the ion exchange in the second step is lesslikely to occur, whereby a desired surface compressive stress anddesired strength when a fracture probability is 1% cannot be obtained.Additionally, if the proportion P is too low, a deeper compressivestress layer may be formed through the first step. This also adverselyaffects the cutting easiness of the glass. Accordingly, in the method ofmanufacturing the cover glass for display devices of the presentinvention, the proportion P is preferably 20 to 40 mol %.

If the proportion Q in the second salt is more than 2 mol %, sufficientamount of K ions are not introduced into the surface layer of the glassin the second step, and the K ions are not sufficiently redispersed inthe surface layer. As a result, a desired surface compressive stress anddesired strength when a fracture probability is 1% cannot be obtained.Accordingly, in the method of manufacturing the cover glass for displaydevices of the present invention, the proportion Q is preferably 0 to 2mol %.

In the method of manufacturing the cover glass for display devices ofthe present invention, the temperature of the first salt is preferably0.8 to 1.05 times the strain point of the glass plate before the ionexchange.

The strain point is a temperature at which or below which glass does notundergo viscous flow. However, in the chemical strengthening, slightviscous flow occurred at a temperature equal to or lower than the strainpoint needs to take into consideration because rearrangement of thestructure at the atomic level substantially affects the results of thechemical strengthening. The effect of decreasing the virtualtemperature, that is, the effect of making the glass structure densetends to be enhanced as the temperature in the first step (temperatureof the first salt) approaches to the strain point. However, at too higha first salt temperature, the surface of the glass is likely to becomecloudy, which may affect the improvement in the reliability of the glassstrength. In addition, a deeper compressive stress layer may be formed,which may adversely affect the cutting easiness of the glass. Further,the compressive stress generated in the first step tends to relax. Onthe contrary, at too low a first salt temperature, ion exchange in thefirst step is not accelerated, and a compressive stress layer having adesired depth is not formed. Further, the structure of the glass is lesslikely to be thermally rearranged, and the effect of modifying thestructure of the surface of the glass is less likely to be obtained.

In the method of manufacturing the cover glass for display devices ofthe present invention, the temperature of the second salt is preferablylower than the temperature of the first salt.

Too high a temperature in the second step (temperature of the secondsalt) tends to relax the compressive stress generated in the first step,in the second step. Further, it tends to form a deeper compressivestress layer, which may adversely affect the cutting easiness of theresulting glass. On the contrary, at too low a second salt temperature,the ion exchange in the second step may not be accelerated.Consequently, a high surface compressive stress may not be generated inthe second step, and the K ions are less likely to be redispersed.Therefore, a compressive stress layer having a desired depth and desiredstrength when a fracture probability is 1% may not be obtained.

Advantageous Effects of Invention

The cover glass for display devices of the present invention hasexcellent cutting processability, and excellent reliability ofmechanical strength, that is, high strength when a cumulative fractureprobability is 1%.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a cover glass for display devices of thepresent invention.

FIG. 2 is a cross-sectional view showing side surface parts of a coverglass for display devices of the present invention.

FIG. 3 is a graph of a calibration curve of an applied load and agenerated stress in Example 1.

FIG. 4 is a Weibull plot showing the relationship between a fracturestress and a cumulative fracture probability.

DESCRIPTION OF EMBODIMENTS

The following description is offered to specifically illustrate anembodiment of the present invention. It should be noted that the presentinvention is not limited only to this embodiment, and the embodiment canbe appropriately altered within the scope of the present invention.

A cover glass for display devices and a glass plate for a cover glassfor display devices according to one embodiment of the present inventioninclude a chemically strengthened glass manufactured by ion exchange ofa surface layer of a glass to replace Na ions in the surface layer ofthe glass with K ions present outside the glass.

The term “cover glass for display devices” according to one embodimentof the present invention herein is not limited to only those used alone,and is intended to also include, for example, cover glasses havingfunctions of a cover and a substrate in one cover glass, used as touchsensor substrates (e.g. cover glasses called “One Glass Solution” or“integrated cover glasses”).

The cover glass is explained based on FIGS. 1 and 2 below. The coverglass for display devices according to one embodiment of the presentinvention has main surface parts 1 where a compressive stress layer isformed and side surface parts (edge parts) 2 composed of regions where acompressive stress layer is formed and regions where no compressivestress layer is formed. According to the present invention, coverglasses are obtained by preliminary performing ion exchange of a glassplate for a cover glass for display devices and cutting the glass plate.For example, the glass plate is a glass plate larger than a desiredcover glass, and all the main surface parts and all the side surfaceparts of the glass plate are chemically strengthened before cutting.This chemically strengthened glass plate can be cut into a plurality ofcover glasses by the cutting. Thus, a plurality of cover glasses can beefficiently produced at the same time from a single large glass plate.The cover glasses obtained by cutting a glass plate have side surfaceparts 2 composed of regions 2 a where a compressive stress layer isformed and regions 2 b where no compressive stress layer is formed.

The region 2 a where a compressive stress layer is formed is continuouswith the main surface part 1, and the region 2 b where no compressivestress layer is formed is continuous with the regions 2 a where acompressive stress layer is formed. That is, the region 2 b where nocompressive stress layer is formed in the side surface part 2 is locatedbetween the regions 2 a where a compressive stress layer is formed onthe main surface parts 1. In such a structure, the region 2 b where nocompressive stress layer is formed in the side surface part 2 isprotected between the regions 2 a where a compressive stress layer iscontinuously formed from the main surface parts 1. Such side surfaceparts 2, of the cover glass, where no compressive stress is formed arenot exposed. Therefore, the mechanical strength of the side surfaceparts 2 can be sufficiently maintained.

The side surface parts 2 of the cover glass are preferably faces formedby physical processing (not only cutting or breaking, but alsochamfering) such as laser scribing, mechanical scribing, and brushpolishing, or chemical processing (chemical cutting) using ahydrofluoric acid solution.

The main surface parts 1 of the cover glass for display devicesaccording to one embodiment of the present invention may be providedwith anti-fingerprint properties, anti-glare properties, or desiredfunctions by surface coating with a chemical, microprocessing, attachinga film to the surface, or the like. Alternatively, on the main surfacepart, an indium tin oxide (ITO) membrane and then a touch sensor may beformed, or printing may be performed according to the color of thedisplay devices. The main surface parts 1 may be partially subjected toa processing for making holes or the like. The shape and size of thesecover glasses may not be limited to simple rectangular shapes, andvarious shapes according to the designed shape of the display devicesare acceptable such as processed rectangular shapes with round corners.

In the cover glass for display devices according to one embodiment ofthe present invention, the depth of the compressive stress layer formedin the main surface part 1 and the region 2 a where a compressive stresslayer is formed in the side surface part 2 is 6 to 15 μm, preferably 8to 13 μm, and more preferably 9 to 12 μm in view of damage resistanceand cutting processability.

The depth of the compressive stress layer formed through ion exchangeherein is measured by photoelasticity using a surface stress meter basedon an optical waveguide effect. The measurement using a surface stressmeter requires the refraction index and photoelasticity constantaccording to the glass composition of the glass before ion exchange. Thesame shall apply to the measurement of a surface compressive stress.

In the cover glass for display devices according to one embodiment ofthe present invention, a shape parameter determined in accordance withJIS R 1625 (1996) based on analysis of a facture stress of the coverglass measured by the coaxial double ring test is not less than 7, andstrength when a cumulative fracture probability is 1% is not less than450 MPa.

The sizes and shapes of a specimen, a load ring, and a support ring, anda loading rate in the coaxial double ring test are given according toISO/DIS (EN) 1288-1 and 1288-5. A calculation method of a fracturestress according to the ISO/DIS (EN) is a method for testing a glasswith small warp generated by the coaxial double ring test. Therefore,the cover glass for display devices of the present invention tends to beovervalued by the calculation method. For this reason, conversion of abreakage load into a fracture stress in this specification should beperformed, instead of by the above calculation method, in the followingways. An actual tensile principal stress generated at a crack of a glassis actually measured or dynamic simulation using a finite element methodfor nonlinear problems is performed.

Weibull statistical analysis of the resulting fracture strength obtainedby the coaxial double ring test is performed in accordance with JISR1625 (1996). That is, a shape parameter of two-parameter Weibulldistribution function (Weibull modulus) and a scale parameter arecalculated by the maximum likelihood estimation. The strength when acumulative fracture probability is 1% is calculated based on theseparameters. The statistical processing is performed based on theresulting fracture strength of desirably not less than 15, moredesirably not less than 20, and still more desirably not less than 30. AWeibull plot is prepared using median rank in accordance with the JIS.

In the cover glass for display devices according to one embodiment ofthe present invention, the shape parameter is not less than 7 and thestrength when a cumulative fracture probability is 1% is not less than450 MPa, and the shape parameter is preferably not less than 8 and thestrength when a cumulative fracture probability is 1% is preferably notless than 500 MPa. A larger strength is better, but strength of 700 MPaor 650 MPa provides sufficiently high strength reliability.

In the cover glass for display devices according to one embodiment ofthe present invention, the surface compressive stress of the mainsurface part of the glass is preferably not less than 450 MPa, morepreferably not less than 550 MPa, and in view of resistance to impactsand damages to the glass, still more preferably not less than 650 MPa. Ahigher surface compressive stress is preferable, and the upper limit maybe 850 MPa, 800 MPa, or 750 MPa.

In the cover glass for display devices according to one embodiment ofthe present invention, the chemically strengthened glass preferably hasa Vickers hardness of 5.0 to 6.0 GPa, more preferably 5.2 to 6.0 GPa,and still more preferably 5.2 to 5.8 GPa. A glass having a Vickershardness of less than 5.0 GPa has poor damage resistance, and thereforecannot withstand commercial use. On the other hand, a glass having aVickers hardness of more than 6.0 GPa is difficult to cut.

In the cover glass for display devices according to one embodiment ofthe present invention, the glass before the ion exchange is made ofsoda-lime glass substantially composed of SiO₂: 65 to 75%, Na₂O+K₂O: 5to 20%, CaO: 2 to 15%, MgO: 0 to 10%, and Al₂O₃: 0 to 5% on a massbasis.

The expression “Na₂O+K₂O: 5 to 20%” herein means that the proportionalamount of Na₂O and K₂O in total in the glass is 5 to 20% by mass.

SiO₂ is a major constituent of glass. If the proportional amount of SiO₂is less than 65%, the glass has reduced strength and poor chemicalresistance. On the other hand, if the proportional amount of SiO₂ ismore than 75%, the glass becomes a highly viscous melt at hightemperatures. Such a glass is difficult to form into a shape.Accordingly, the proportional amount should be in the range of 65 to75%, and preferably 68 to 73%.

Na₂O is an essential component that is indispensable for the chemicalstrengthening treatment. If the proportional amount of Na₂O is less than5%, sufficient ions are not exchanged, namely, the chemicallystrengthening treatment does not improve the strength very much. On theother hand, if the proportional amount is more than 20%, the glass mayhave poor chemical resistance and poor weather resistance. Accordingly,the proportional amount should be in the range of 5 to 20%, preferably 5to 18%, and more preferably 7 to 16%. K₂O is not an essential component,but acts as a flux for the glass together with Na₂O upon melting theglass, and acts also as an adjunct component for accelerating ionexchange when added in a small amount. However, when excessive K₂O isused, K₂O produces a mixed alkali effect with Na₂O to inhibit movementof Na⁺ ions. As a result, the ions are less likely to be exchanged. Ifthe proportional amount of K₂O is more than 5%, the strength is lesslikely to be improved by ion exchange. Accordingly, the proportionalamount is preferably not more than 5%. The proportional amount ofNa₂O+K₂O is 5 to 20%, preferably 7 to 18%, and more preferably 10 to17%.

CaO improves the chemical resistance of the glass, and additionallyreduces the viscosity of the glass in the molten state. For the purposeof improving the mass productivity of the glass, CaO is preferablypresent in an amount of not less than 2%. However, if the proportionalamount exceeds 15%, it acts to inhibit movement of Na⁺ ions.Accordingly, the proportional amount should be in the range of 2 to 15%,preferably 4 to 13%, and more preferably 5 to 11%.

MgO is not an essential component, but is preferably used in place of aportion of CaO because it is less likely to inhibit movement of Na⁺ ionsthan CaO. MgO, however, is not as effective as CaO in reducing theviscosity of the glass in the molten state. When MgO is used in anamount of more than 10%, it allows the glass to become highly viscous,which is a contributing factor to poor mass productivity of the glass.Accordingly, the proportional amount should be in the range of 0 to 10%,preferably 0 to 8%, and more preferably 1 to 6%.

Al₂O₃ is not an essential component, but improves the strength and theion exchange capacity. If the proportional amount of Al₂O₃ is more than5% on amass basis, the glass becomes a highly viscous melt at hightemperatures, and additionally is likely to be devitrified. Such a glassmelt is difficult to form into a shape. Moreover, the ion exchangecapacity is increased too much, and therefore a deep compressive stressmay be formed. As a result, the chemical strengthening may make theglass difficult to cut. Accordingly, the proportional amount should bein the range of 0 to 5%, preferably 1 to 4%, and more preferably 1 to 3%(not including 3).

The glass before the ion exchange is made of soda-lime glass andsubstantially composed of the above components, but may further containsmall amounts, specifically up to 1% in total, of other components suchas Fe₂O₃, TiO₂, CeO₂, and SO₃.

The base glass before the ion exchange preferably has a strain point of450 to 550° C., and more preferably 480 to 530° C. If the glass has astrain point of lower than 450° C., it does not have heat resistancehigh enough to withstand the chemical strengthening. On the other hand,if the strain point is higher than 550° C., the glass has too high amelting temperature, which means that such glass plates cannot beproduced efficiently and increase costs.

The glass before the ion exchange is preferably one formed by commonglass forming processes such as a float process, a roll-out process, anda down-draw process. Among these, one formed by a float process ispreferable. A glass may be a glass prepared by etching a glass baseplate using a hydrofluoric acid solution by a usual method.

The shape of the base glass before the ion exchange is not particularlylimited, and is preferably a plate shape. In the case where the glasshas a plate shape, it may be a flat plate or a warped plate, and variousshapes are included within the scope of the present invention. Shapessuch as rectangular shapes and disc shapes are included within thedefinition of the flat plate in the present invention, and rectangularshapes are preferable among others.

In the cover glass for display devices according to one embodiment ofthe present invention, the glass is preferably as thin as possible toreduce the weight of final products (e.g. mobile products) and ensurethe space for batteries or other components in device products.Unfortunately, however, too thin a glass plate may generate a largestress when it warps. On the other hand, too thick a glass plateincreases the weight of final device products and degrades thevisibility of display devices. The upper limit of the thickness of theglass is preferably 3 mm, more preferably 2 mm, still more preferably1.8 mm, and particularly preferably 1.1 mm. The lower limit of thethickness of the glass is preferably 0.05 mm, more preferably 0.1 mm,still more preferably 0.2 mm, and particularly preferably 0.3 mm.

In a method of manufacturing the cover glass for display devicesaccording to one embodiment of the present invention, the ion exchangeincludes a first step of contacting the glass plate with a first saltthat includes Na ions and K ions at a proportion P of the Na ions asexpressed as a molar percentage of the total amount of the Na ions andthe K ions; and a subsequent second step of contacting the glass platewith a second salt that includes Na ions and K ions at a proportion Q ofthe Na ions as expressed as a molar percentage of the total amount ofthe Na ions and the K ions, where the proportion Q is smaller than theproportion P.

Use of such a first salt allows the surface layer of the glass to bemodified so that the layer contains both Na ions and K ions, in thefirst step. It is assumed that the virtual temperature of the glassdecreases to make the structure of the glass tight (dense) in this firststep. In the subsequent second step, ion exchange between Na ions and Kions occurs in the outermost surface of the glass, and the K ions arethree-dimensionally redispersed in the surface layer of the modifiedglass. It is assumed that a high surface compressive stress generatesand microcracks on the surface of a glass are prevented from growing inthis second step.

Further, since the composition of the surface layer of the glass ismodified through the first step, the relaxation of the compressivestress generated in the second step is prevented. That is, the surfacecompressive stress generated by ion exchange in the second step isslightly relaxed and mostly left because the glass article has alreadybeen subjected to the first step. Therefore, a high surface compressivestress can be obtained.

The expression “contacting a glass plate with a salt” used for the firstand second steps means to contact the glass plate with a salt bath orsubmerge the glass plate in a salt bath. Thus, the term “contact” usedherein is intended to include “submerge” as well.

The contact with a salt can be accomplished by, for example, directlyapplying the salt in a paste form to the glass plate, spraying the saltin an aqueous solution form, submerging the glass plate in a molten saltheated to its melting point or higher. Among these, submerging in amolten salt is preferable.

The salt may be one of or a mixture of two or more of nitrates,sulfates, carbonates, hydroxide salts, and phosphates. A salt containingNa ions may preferably be a sodium nitrate molten salt. A saltcontaining K ions may preferably be a potassium nitrate molten salt.Therefore, a salt containing Na ions and K ions may preferably be amolten salt composed of a mixture of sodium nitrate and potassiumnitrate.

The depth of the compressive stress layer formed through the first stepis preferably 5 to 23 μm, as described above. The depth is morepreferably 7 to 20 μm and still more preferably 10 to 18 μm. In order tomake a compressive stress layer with the above depth, the temperature ofthe first salt and the period of time for contact of the glass platewith the first salt are controlled depending on the proportion P.

In order to make a compressive stress layer with a depth of 6 to 15 μmthrough the second step, the temperature of the second salt and theperiod of time for contact of the glass plate with the second salt arecontrolled depending on the proportion Q.

If the proportion P in the first salt is too high, the surface of theglass plate is likely to become cloudy, which avoids improvement inreliability of the glass strength. On the contrary, if the proportion Pin the first salt is too low, the composition of the surface layer ofthe glass plate tends to be too much modified, and most Na ions in theglass are ion exchanged with K ions in the first step. As a result, ionexchange in the second step may not be accelerated, failing to give adesired surface compressive stress and desired strength when a fractureprobability is 1%. If the proportion P is too low, a deeper compressivestress layer tends to be formed through the first step. This alsoadversely affects the cutting easiness of the resulting glass.Therefore, the proportion P is preferably 20 to 40 mol % and morepreferably 25 to 35 mol %.

If the proportion Q in the second salt is more than 2 mol %, sufficientK ions may not be introduced into the surface layer of the glass in thesecond step, and driving force of redispersion of K ions tends to beweakened. Therefore, a desired surface compressive stress and desiredstrength when a fracture probability is 1% cannot be obtained. Theproportion Q is preferably 0 to 2 mol % and more preferably 0 to 1 mol%. Thus, the second salt may contain only K ions as a cation, and maynot substantially contain Na ions.

Although the first and second salts are each a pure salt of Na ionsand/or K ions in the above description, this embodiment does notpreclude the presence of stable metal oxides, impurities, and othersalts that do not react with the salts, provided that they do not impairthe purpose of the present invention. For example, the first or secondsalt may contain Ag ions or Cu ions as long as the proportion Q is inthe range of 0 to 2 mol %.

At too high a temperature in the first step (temperature of the firstsalt), the surface of the glass tends to become cloudy, which avoidsimprovement of the reliability of the glass strength. Further, a deepercompressive stress layer may be formed, which may adversely affect thecutting easiness of the resulting glass. The compressive stressgenerated in the first step tends to relax. On the contrary, at too lowa first salt temperature, ion exchange in the first step may not beaccelerated, which avoids formation of a compressive stress layer havinga desired depth. Further, the glass structure is less likely to bethermally rearranged, and the effect of modifying due to the virtualtemperature of the glass is less likely to be obtained, whereby thestructure of the glass does not become tight. The temperature of thefirst salt is preferably 0.8 times to 1.05 times the strain point of theglass plate before the ion exchange, more preferably 0.83 times to 1.0time, and still more preferably 0.87 times to 1.0 time.

Chemical strengthening is intended to generate a compressive stressusing a difference between ionic radii of two types of ions replacedeach other by ion exchange. Therefore, the temperature of the first saltshould not exceed the annealing point of the glass plate before the ionexchange.

Too high a temperature in the second step may relax, in the second step(temperature of the second salt), the compressive stress generated inthe first step. In addition, a deeper compressive stress layer may beformed, which may adversely affect the cutting easiness of the resultingglass. On the other hand, too low a second salt temperature fails toaccelerate ion exchange in the second step. Consequently, a high surfacecompressive stress may not be generated in the second step, and K ionsare less likely to be redispersed, failing to give a compressive stresslayer having a desired depth and desired strength when a fractureprobability is 1%. Therefore, the temperature of the second salt ispreferably the same or lower than the temperature of the first salt, andmore preferably lower than the temperature of the first salt. Further,the temperature of the second salt is preferably not less than 390° C.,more preferably not less than 400° C., and still more preferably notless than 420° C.

A total time period of the contact of the glass plate with the firstsalt in the first step and the contact of the glass plate with thesecond salt in the second step is preferably 1 to 12 hours and morepreferably 2 to 6 hours.

Specifically, too long a contact of the glass plate with the first salttends to relax the compressive stress generated in the first step, andadditionally tends to provide a deeper compressive stress layer. Thisadversely affects the cutting easiness of the resulting glass. On theother hand, too short a contact of the glass plate with the first saltmay not produce a sufficient effect of modifying the surface layer ofthe glass in the first step, and therefore tends to cause stressrelaxation in the second step.

Therefore, the time period of the contact of the glass plate with thefirst salt in the first step is preferably 0.5 to 8 hours, morepreferably 1 to 6 hours, and still more preferably 1 to 4 hours.

In the second step, it is preferable to reduce the relaxation of thestress generated by the ion exchange to a minimum. However, a longercontact of the glass plate with the salt increases the relaxation of thestress. Additionally, a longer contact tends to provide a deepercompressive stress layer in the second step. This also adversely affectsthe cutting easiness of the resulting glass. On the other hand, tooshort a contact of the glass plate with the second salt fails to allowthe alkali metal ions A and the alkali metal ions B to be exchangedsufficiently, and therefore a desired compressive stress may not begenerated.

Therefore, the time period of the contact of the glass plate with thesecond salt in the second step is preferably 0.5 to 8 hours, morepreferably 0.5 to 6 hours, and still more preferably 0.5 to 3 hours.

EXAMPLES

The following examples are offered to more specifically illustrate theembodiment of the present invention. It should be noted that the presentinvention is not limited only to these examples.

Example 1 (1) Preparation of Chemically Strengthened Glass andEvaluation of Surface Compressive Stress and Depth of Compressive StressLayer

As a glass plate before ion exchange (chemical strengthening), a 0.7-mmthick soda-lime glass plate with 400 mm×500 mm sizes (SiO₂: 71.6%, Na₂O:12.5%, K₂O: 1.3%, CaO: 8.5%, MgO: 3.6%, Al₂O₃: 2.1%, Fe₂O₃: 0.10%, SO₃:0.3% (on a mass basis) the strain point of the plate glass was 503° C.)(hereinafter, referred to as glass base plate) was produced by a floatprocess.

The glass base plate prepared above was submerged in a molten salt(first salt, proportion P: 34.7 mol %) bath composed of a mixture of65.3 mol % of potassium nitrate and 34.7 mol % of sodium nitrate at aconstant temperature of 475° C. for 120 minutes, as a first step. Theglass base plate was then taken out from the bath and gradually cooled,and the surface of the glass base plate was washed and dried.

In a subsequent second step, the dried glass base plate was submerged ina molten salt (second salt, proportion Q: 0.1 mol %) bath substantiallycomposed of a mixture of 99.9 mol % of potassium nitrate and 0.1 mol %of sodium nitrate at a constant temperature of 435° C. for 60 minutes.The glass base plate was then taken out from the bath and graduallycooled, and the surface of the glass base plate was washed and dried.

The obtained chemically strengthened glass base plate was measured forthe surface compressive stress and the depth of the compressive stresslayer formed at the surface of the glass using a surface stress meter(FSM-60V, produced by Toshiba Glass Co., Ltd. (currently OriharaIndustrial Co., Ltd.)). The refraction index and photoelasticityconstant of the glass composition of the soda-lime glass used for themeasurement with the surface stress meter were 1.52 and 26.8((nm/cm)/MPa), respectively. The results of the measurement show thatthe surface compressive stress was 675 MPa and the depth of thecompressive stress layer was 12 μm. The depth of the compressive stresslayer formed through the first step was 14 μm.

(2) Cutting of Chemically Strengthened Glass

The chemically strengthened glass base plate was cut into a plurality of66-mm-square glasses as shown in FIGS. 1 and 2 using a mechanicalscriber having a carbide wheel glass cutter.

(3) Evaluation of Strength Reliability (3-1) Measurement of FractureStress by Coaxial Double Ring Test

Each chemically strengthened cut glass was measured for a fracturestress.

The sizes and shapes of a specimen, a load ring, and a support ring, anda loading rate in the coaxial double ring test are given according toISO/DIS (EN) 1288-1 (Glazing in Building-Determination of the bendingstrength of glass—Part 1: Fundementals of testing glass) and 1288-5(Part 5: Coaxial double ring test on flat specimens with small or mediumtest surface areas). Specifically, a 66-mm square specimen was used, anda load ring having a radius of 6 mm and a support ring having a radiusof 30 mm were used, and a loading rate was set to 1.6 to 2.4 MPa/sec.The loading rate varied depending on warp generated in a glass specimenduring the coaxial double ring test, and a crosshead speed was thereforecontrolled so that the loading rate when the glass breaks satisfies theabove range.

The calculation method of a fracture stress according to the ISO/DIS(EN) is a method for testing a glass with small warp generated by thecoaxial double ring test. Therefore, the cover glass for display devicesof the present invention having a high strength tends to be overvaluedbecause of the high warp of the cover glass. In the present example,rosette analysis was performed using triaxial strain gauges to determinea principal stress generated at a broken part of the cover glass. Acalibration curve for converting a load (applied load) applied to thecover glass into the principal stress (generated stress) was prepared.In this test, most of the specimens were broken at a part just below aload ring. Therefore, the calibration curve prepared is as for the partjust below a load ring. Specimens broken at the part were counted as“available”. FIG. 3 shows a calibration curve as for a part just below aload ring.

In the above procedures, the fracture stresses (surface strength) of thechemically strengthened cut glasses were measured by the coaxial doublering test. 56 specimens are available.

(3-2) Evaluation of Shape Parameter and Strength when FractureProbability is 1%

Statistical analysis of the resulting strength was performed by maximumlikelihood estimation in accordance with JIS R1625 (1996) (Weibullstatistics of strength data for fine ceramics). The results show that ashape parameter m (Weibull modulus) was 7.43 and a scale parameter σ₀was 919 MPa. The shape parameter and the scale parameter were written tothree significant figures in accordance with the JIS. The fracturestress when a cumulative fracture probability was 1%, that is, thestrength when a cumulative fracture probability was 1% was calculatedbased on these parameters to be 495 MPa. FIG. 4 shows a Weibull plotshowing the relationship between a cumulative fracture probability (%)and a fracture stress (MPa) (FIG. 4 also shows Weibull plots ofComparative Examples 2 to 4 and Reference Example 1). The resultingstrengths were ranked using median rank.

Example 2

Chemically strengthened cut glasses were prepared as in Example 1 exceptthat the thickness of the glass base plate was changed and thetemperature of the first salt was set at 470° C., and evaluated. Thedepth of the compressive stress layer formed through the first step, thesurface compressive stress, and the depth of the compressive stresslayer of each glass were measured as in Example 1 to be 12 μm, 683 MPa,and 10 μm, respectively. The shape parameter and the strength when acumulative fracture probability was 1% determined as in Example 1 were8.07 and 512 MPa, respectively.

Example 3

Chemically strengthened cut glasses were prepared as in Example 2 exceptthat the thickness of the glass base plate was changed, and evaluated.The depth of the compressive stress layer formed through the first step,the surface compressive stress, and the depth of the compressive stresslayer of each glass were measured as in Example 1 to be 12 μm, 677 MPa,and 11 μm, respectively. The shape parameter and the strength when acumulative fracture probability was 1% determined as in Example 1 were11.5 and 578 MPa, respectively.

Example 4

Chemically strengthened cut glasses were prepared as in Example 2 exceptthat a glass with an outermost surface etched tens of micrometers deepusing a hydrofluoric acid solution was used as the glass base platebefore ion exchange (chemical strengthening), and evaluated. The depthof the compressive stress layer formed through the first step, thesurface compressive stress, and the depth of the compressive stresslayer of each glass were measured as in Example 1 to be 12 μm, 665 MPa,and 10 μm, respectively. The shape parameter and the strength when acumulative fracture probability was 1% determined as in Example 1 were9.21 and 538 MPa, respectively.

Example 5

A glass base plate was prepared as in Example 2 except that thethickness of the glass base plate was changed. The glass base plateprepared above was submerged in a molten salt (first salt, proportion P:20.0 mol %) bath composed of a mixture of 80.0 mol % of potassiumnitrate and 20.0 mol % of sodium nitrate at a constant temperature of485° C. for 120 minutes, as a first step. Other operations wereperformed as in Example 1. In a subsequent second step, the resultingglass base plate was submerged in a molten salt (second salt, proportionQ: 0.0 mol %) bath substantially composed of 100.0% potassium nitrate ata constant temperature of 450° C. for 60 minutes. Other operations wereperformed as in Example 1.

Chemically strengthened cut glasses of Example 5 were prepared as inExample 1. The surface compressive stress and the depth of thecompressive stress layer of each glass were 680 MPa and 13 μm,respectively. The depth of the compressive stress layer formed throughthe first step was 15 μm. The shape parameter and the strength when acumulative fracture probability was 1% determined as in Example 1 were12.1 and 575 MPa, respectively.

In Table 1, the surface compressive stresses, the depths of thecompressive stress layers, the depths of the compressive stress layersformed through the first step, the shape parameters, the scaleparameters, and the strengths when a cumulative fracture probability was1% of the cover glasses for display devices of Examples 1 to 5 werelisted.

TABLE 1 Example Example Example Example Example Comparative ComparativeComparative Comparative Reference 1 2 3 4 5 Example 1 Example 2 Example3 Example 4 Example 1 Composition Soda Soda Soda Soda Soda Soda SodaSoda Soda Aluminosilicate lime lime lime lime lime lime lime lime limeThickness (mm) 0.7 0.5 1.1 0.7 1.1 0.7 0.7 0.7 0.5 0.7 Surface 675 683677 665 680 470 619 525 650 compressive stress (MPa) Depth of 12 10 1110 13 12 12 17 42 compressive stress layer (μm) Depth of 14 12 12 12 15compressive stress layer formed through first step (μm) Number of 56 3030 32 18 29 30 30 30 30 available samples Shaped parameter 7.43 8.0711.5 9.21 12.1 2.42 3.55 3.92 5.47 7.37 Scale parameter 919 905 864 886840 323 731 896 690 948 (MPa) Strength when 495 512 578 538 575 48 200278 298 508 cumulative fracture probability is 1% (MPa)

As shown in Table 1, the chemically strengthened cut glasses of Examples1 to 5 each have a shape parameter of not less than 7 and strength whena cumulative fracture probability is 1% of not less than 450 MPa, whichindicates that the chemically strengthened cut glasses have excellentsurface-strength reliability. This is because the surface layer of theglass is more uniformly chemically strengthened by performing ionexchange including the first and second steps, and the glass has a highsurface compressive stress.

The chemically strengthened cut glasses evaluated in Examples 1 to 5have both cutting processability and strength reliability, and aretherefore suitable for a cover glass having a shape shown in FIGS. 1 and2 obtained by cutting a chemically strengthened mother glass.

Comparative Example 1

A glass base plate was prepared as in Example 1. The glass base platewas cut into cut glasses as in Example 1 before the glass base plate waschemically strengthened. The coaxial double ring test was performed oneach cut glass as in Example 1 to evaluate the strength properties. As aresult, the shape parameter and the strength when a cumulative fractureprobability was 1% were 2.42 and 48 MPa, respectively. The cut glasseswere not suitable for a cover glass for display devices because of itspoor strength reliability.

Comparative Example 2

A glass base plate was prepared as in Example 1. The glass base platewas submerged in a molten salt containing about 97 mol % of potassiumnitrate at a constant temperature of 475° C. for 70 minutes to prepare achemically strengthened glass base plate. In Comparative Example 2, awidely known chemical strengthening including “submerging a glass baseplate in a molten salt (generally containing impurities) substantiallycomposed of a potassium nitrate molten salt” was used instead of the ionexchange including the first and second steps described in Examples 1 to5. The surface compressive stress and the depth of the compressivestress layer were measured to be 470 MPa and 12 μm, respectively.

Chemically strengthened cut glasses were prepared from a chemicallystrengthened glass base plate as in Example 1. Evaluation of thestrength by the coaxial double ring test showed that the shape parameterand the strength when a cumulative fracture probability was 1% were 3.55and 200 MPa, respectively.

Comparative Example 3

A glass base plate was prepared as in Example 1. The glass base platewas submerged in a molten salt containing about 99.5 mol % of potassiumnitrate at a constant temperature of 465° C. for 90 minutes to prepare achemically strengthened glass base plate. The surface compressive stressand the depth of the compressive stress layer were measured as inExample 1 to be 619 MPa and 12 μm, respectively.

The chemically strengthened glass base plate was cut into glasses, andthe strength of each glass was evaluated by the coaxial double ring testas in Example 1. As a result, the shape parameter and the strength whena cumulative fracture probability was 1% were 3.92 and 278 MPa,respectively.

Comparative Example 4

A glass base plate was prepared as in Example 1 except that thethickness of the glass base plate was changed. The glass base plateprepared above was submerged in a molten salt (first salt, proportion:34.7 mol %) bath composed of a mixture of 34.7 mol % of potassiumnitrate and 65.3 mol % of sodium nitrate at a constant temperature of505° C. for 120 minutes, as a first step. Other operations wereperformed as in Example 1. In a subsequent second step, the resultingglass base plate was submerged in a bath (second salt, proportion Q: 0.1mol %) substantially composed of a mixture of 99.9 mol % of potassiumnitrate and 0.1 mol % of sodium nitrate at a constant temperature of495° C. for 60 minutes. Other operations were performed as in Example 1.The surface compressive stress and the depth of the compressive stresslayer of the obtained chemically strengthened glass base plate weremeasured to be 525 MPa and 17 μm, respectively. The temperatures of thesalts in the first and second steps were higher than those of the saltsused in Example 1. Therefore, the relaxation of the generatedcompressive stress tended to accelerate and a deeper compressive stresslayer tended to be formed.

The chemically strengthened glass base plate was able to be cut intoglasses as in Example 1, but the yield of the glasses were slightlylower than that in Example 1. This is because the compressive stresslayer, which is deeper than that in Example 1, prevents formation of ascribe line in the glass base plate.

The shape parameter and the strength when a cumulative fractureprobability was 1% determined as in Example 1 were 5.47 and 298 MPa,respectively.

In Table 1, the surface compressive stresses, the depths of thecompressive stress layers, the shape parameters, the scale parameters,and the strengths when a cumulative fracture probability was 1% of thecover glasses for display devices of Comparative Examples 1 to 4 werelisted. Weibull plots of Comparative Examples 2 to 4 were shown in FIG.4.

The cover glass of Comparative Example 2 was prepared only throughone-step chemical strengthening. Therefore, the surface layer of theglass was not uniformly ion exchanged, and had a lower surfacecompressive stress as compared to the cover glasses of Examples 1 to 5.For this reason, such a cover glass may not have excellent strengthreliability.

The surface compressive stress of the cover glass of Comparative Example3 was higher than that in Comparative Example 2, and approaches thesurface compressive stresses in Examples 1 to 5. However, since thecover glass of Comparative Example 3 was prepared only through one-stepchemical strengthening, the uniformity of ion exchange of the surfacelayer was poor, and excellent strength reliability was not achieved.

The cover glass of Comparative Example 4 was prepared through the firstand second steps, but tends to have a deeper compressive stress layerthan those in Examples 1 to 5, which is accompanied by relaxation of thesurface compressive stress. Therefore, excellent strength reliabilitywas not achieved in such a cover glass. Further, it is more difficult tocut the chemically strengthened glass than those in Examples 1 to 5.

Reference Example 1

An aluminosilicate base glass plate generally regarded to be suitablefor chemical strengthening was submerged in a molten salt substantiallycomposed only of 100% of potassium nitrate at a constant temperature of470° C. for 75 minutes to prepare a chemically strengthened glass plate.The surface compressive stress and the depth of the compressive stresslayer of the chemically strengthened glass plate were measured to be 652MPa and 33 μm, respectively. In the measurement, the refraction index of1.51 and the photoelasticity constant of 29.4 ((nm/cm)/MPa) as theproperties of aluminosilicate were used.

The chemically strengthened glass base plate was not able to be cutunlike the glass of Example 1 because any crack for scribing was notformed on the glass, or even if a crack was able to be formed on theglass, the glass did not split along the crack, that is, broke along anunintended direction. Further, since the chemically strengthened glasswas not able to be cut, a cut glass of the chemically strengthened glasswith a shape shown in FIGS. 1 and 2 was not obtained, and the strengthof a chemically strengthened cut glass was unmeasurable.

Therefore, chemical strengthening was performed in a manner differentfrom the manner in Example 1 in which a chemically strengthened singlelarge glass plate was cut into cut glasses. That is, a glass base platewas cut into cut glasses first before it was chemically strengthened,and the cut glasses were then ion exchanged. Specifically, thealuminosilicate glass not ion exchanged at this time for chemicalstrengthening was cut into 66-mm square glasses before ion exchange asin Example 1. The glasses were then chemically strengthened bysubmerging them into a molten salt bath substantially composed of 100mol % potassium nitrate at a constant temperature of 480° C. for 90minutes. The surface compressive stress and the depth of the compressivestress layer of the obtained chemically strengthened glass base platewere measured to be 650 MPa and 42 μm, respectively. Through thesesteps, chemically strengthened cut glasses of Reference Example 1 wereprepared. Unlike Examples 1 to 5, a compressive stress layer was formedin the entire region of each side surface part of each chemicallystrengthened cut glass of Reference Example 1.

It is considered that thus obtained glasses made of aluminosilicateglass for chemical strengthening are suitable for a cover glass in viewof strength reliability because such glasses have a high compressivestress and a deep compressive stress layer. However, suchaluminosilicate glass is too efficiently ion exchanged to beappropriately cut. Therefore, use of aluminosilicate glass is notsuitable for efficient mass production in which small size cover glassesare produced from a chemically strengthened single large glass.

Next, the coaxial double ring test was performed on the chemicallystrengthened cut glass of Reference Example 1 as in Example 1 toevaluate the strength properties. The evaluation showed that the shapeparameter and the strength when a cumulative fracture probability was 1%were 7.37 and 508 MPa, respectively.

Comparison of Examples 1 to 5 and Reference Example 1 (see Table 1 andFIG. 4) showed that although the chemically strengthened glass (coverglass for display devices) of the present invention is a glass made ofcommonly-used soda-lime glass, which is not suitable for chemicalstrengthening in view of composition, the glass has excellent strengthreliability similar to those according to Reference Example 1, andexcellent cutting processability.

REFERENCE SIGNS LIST

-   1 Main surface part of glass-   2 Side surface part of glass-   2 a Region where compressive stress layer is formed-   2 b Region where no compressive stress layer is formed

1. A cover glass for display devices, comprising: a chemicallystrengthened glass manufactured by ion exchange of a surface layer of aglass plate to replace Na ions with K ions, the cover glass having amain surface part in which a compressive stress layer is formed and aside surface part that has a region where the compressive stress layeris formed and a region where no compressive stress layer is formed, thecompressive stress layer having a depth of 6 to 15 μm, wherein a shapeparameter determined in accordance with MS R 1625 (1996) based onanalysis of a facture stress of the cover glass measured by a coaxialdouble ring test is not less than 7, and strength of the cover glasswhen a cumulative fracture probability is 1% is not less than 450 MPa;and the glass plate before the ion exchange is made of soda-lime glasssubstantially composed of SiO₂: 65 to 75%, Na₂O+K₂O: 5 to 20%, CaO: 2 to15%, MgO: 0 to 10%, and Al₂O₃: 0 to 5% on a mass basis.
 2. The coverglass for display devices according to claim 1, wherein the main surfacepart has a surface compressive stress of not less than 450 MPa.
 3. Thecover glass for display devices according to claim 1, wherein the ionexchange includes a first step of contacting the glass plate with afirst salt that includes Na ions and K ions at a proportion P of the Naions as expressed as a molar percentage of the total amount of the Naions and the K ions; and a subsequent second step of contacting theglass plate with a second salt that includes Na ions and K ions at aproportion Q of the Na ions as expressed as a molar percentage of thetotal amount of the Na ions and the K ions, where the proportion Q issmaller than the proportion P.
 4. A glass plate for a cover glass fordisplay devices, comprising: a chemically strengthened glass platemanufactured by ion exchange of a surface layer of a glass plate toreplace Na ions with K ions, the glass plate having a compressive stresslayer with a depth of is 6 to 15 μm, wherein a shape parameterdetermined in accordance with MS R 1625 (1996) based on analysis of afacture stress of the glass plate measured by a coaxial double ring testis not less than 7, and strength of the glass plate when a cumulativefracture probability is 1% is not less than 450 MPa, and the glass platebefore the ion exchange is made of soda-lime glass substantiallycomposed of SiO₂: 65 to 75%, Na₂O+K₂O: 5 to 20%, CaO: 2 to 15%, MgO: 0to 10%, and Al₂O₃: 0 to 5% on a mass basis.
 5. A method of manufacturingthe cover glass for display devices according to claim 1, comprising thesteps of: performing ion exchange of a surface layer of a glass plate toreplace Na ions with K ions; and cutting the glass plate, the ionexchange including a first step of contacting the glass plate with afirst salt that includes Na ions and K ions at a proportion P of the Naions as expressed as a molar percentage of the total amount of the Naions and the K ions; and a subsequent second step of contacting theglass plate with a second salt that includes Na ions and K ions at aproportion Q of the Na ions as expressed as a molar percentage of thetotal amount of the Na ions and the K ions, where the proportion Q issmaller than the proportion P.
 6. The method according to claim 5,wherein the compressive stress layer formed at a surface of the glassplate through the first step has a depth of 5 to 23 μm.
 7. The methodaccording to claim 5, wherein the proportion P is 20 to 40 mol %.
 8. Themethod according to claim 5, wherein the proportion Q is 0 to 2 mol %.9. The method according to claim 5, wherein the temperature of the firstsalt is 0.8 to 1.05 times the strain point of the glass plate before theion exchange.
 10. The method according to claim 5, wherein thetemperature of the second salt is lower than the temperature of thefirst salt.